1. Field of the Invention
The present invention relates to radio frequency (RF) power amplifiers.
2. State of the Art
In many RF communications systems, particularly cellular telephone systems, an RF transmitter portion of an RF transceiver is operated intermittently. In the GSM cellular telephone system, for example, communications are structured on a TDMA (Time Division Multiple Access) basis in which a communications frame comprises some number of timeslots, e.g., eight or sixteen. A particular handset transmits only within its assigned timeslot(s). During the remaining timeslots, the handset does not transmit, meaning that its RF power amplifier is turned off. Hence, the RF power amplifier is repeatedly turned on to transmit during its timeslot and then turned off.
The RF power amplifier is the major consumer of power within a radio transceiver. Unless proper precautions are taken, turning the power amplifier on can create an inrush condition that disturbs other transceiver circuitry. In particular, a PLL (phase lock loop) of the transceiver is susceptible to "frequency pulling" in which turning on the power amplifier disturbs the PLL lock condition. Frequency pulling is not only undesirable but in certain communication systems, such as GSM, is intolerable.
The same problem arises to a lesser degree in connection with power level adjustments. As the distance between a mobile handset and a fixed base station varies, the handset may be commanded to adjust its transmission power upward or downward. Such power control actions may also cause frequency pulling.
Various solutions to the problem of frequency pulling have been proposed.
U.S. Pat. No. 5,424,692 to McDonald, for example, incorporated herein by reference, describes a switchable impedance circuit used at both the input port and the output port of a RF power amplifier. The switchable impedance circuit receives a power down control signal. When the power down control signal is asserted, the switchable impedance circuits switch into alternate impedance states, whereby the "powered down" input impedances at the input and output ports, respectively, appear substantially unchanged in relation to the corresponding "powered up" impedances. In essence, the reference teaches the use of switches in series and shunt along the signal transmission line to selectively connect a through-path or a shunt path.
U.S. Pat. No. 5,774,017 to Adar, incorporated herein by reference, describes an amplifier, operable in two predetermined frequency bands, wherein the input impedance is controlled by using switches to select passive elements in series with the input transmission line. The reference also teaches using shunt switching elements to change interstage matching frequency responses. Since no mention is made of how output power is to be controlled, it appears that the amplifier must operate linearly, with output power being varied by changing the input power applied to the amplifier(s).
There remains a need for an improved power amplifier that further minimizes impedance changes as the power amplifier changes from one operational state to another.